1. Field of the Invention
The present invention relates to a device for converting image formats for display on a television set having a different format than the image format. The present invention more specifically applies to such a device performing a non-linear conversion.
2. Discussion of the Related Art
Television screens are currently available in two standardized formats, respectively 4/3 and 16/9, which define the width of the screen with respect to its height. Images to be displayed on the screen are likely to be broadcast according to these two formats. If the images are received in a format different from that of the screen, the images are generally compressed or expanded prior to their display to avoid having the images significantly deformed upon display.
The distortion (compression or expansion) performed on the images can be performed vertically and/or horizontally. The present invention more specifically relates to a device for converting the image format by performing a horizontal distortion.
A so-called panorama display mode has recently been provided, which consists in performing a horizontal non-linear distortion of the images to be displayed in a different format. This display mode enables display of the image in full screen by minimizing the distortion in the central area of the screen and by enhancing the distortion on the edges. Thus, the appearance of very large black bands on both sides of the images or a loss of the image edges is avoided, while minimizing the distortion which can be perceived by the user.
FIGS. 1A to 1D schematically illustrate the panorama display mode. FIGS. 1A and 1C show, respectively, images in format 4/3 and 16/9. FIGS. 1B and 1D schematically show the display of the images shown, respectively, in FIGS. 1A and 1C, on a screen, respectively, in format 16/9 and in format 4/3. In the representation of FIGS. 1A to 1D, for example, an image defining a triangle has been shown, which constitutes a geometric figure illustrating particularly well the distortion performed in panorama mode.
For the display of an image in format 4/3 (FIGS. 1A and 1B) on a screen at format 16/9, the image is expanded horizontally. Preferably, the central area of the image does not undergo any modification while the expansion becomes greater as one approaches the edges of the image.
For the display of images in format 16/9 on a screen at format 4/3, (FIGS. 1C and 1D), a non-linear compression is performed instead of an expansion.
FIG. 2 shows an example of a conventional non-linear compression digital device.
This device includes two memories 1 and 2 associated with input and output switches, respectively, 3 and 4, for selecting the memory used for writing and that used for reading. Each memory 1 or 2 is capable of storing all the video data relating to one line of the image, that is, both the luminance information (Y) and the chrominance information (for example, B-Y and R-Y). Memories 1 and 2 and switches 3 and 4 are associated with a control circuit 5 for selecting, for each line, which of memories 1 and 2 which is to be in the write mode and which is to be in the read mode. Thus, an image line, for example, in format 4/3, is stored at the rate of a first clock CKW in one of the memories, for example, memory 1, while the content of the other memory, for example memory 2, which is read at the rate of a second clock CKR for displaying this image in format 16/9. Analog-to-digital and digital-to-analog converters (not shown) are respectively associated with the circuit inputs and outputs.
To implement a non-linear distortion while respecting the image rate, clock CKW is usually a fixed-frequency clock and clock CKR is a variable-frequency clock, having its mean frequency on a display line corresponding to the frequency of clock CKW.
Write frequency CKW corresponds to the acquisition frequency of the line pixels while read frequency CKR corresponds to the display frequency of the pixels.
Clock CKR is generally frequency modulated so that, for a linear expansion, its frequency is lower than that of clock CKW at the beginning and at the end of each line, and so that its frequency is slightly higher than that of clock CKW in the central portion of each line. The image thus is, in practice, slightly distorted in its central portion.
A problem which arises in this type of device relates to obtaining write and read clocks, respectively CKW and CKR, of the memories.
Generally, frequencies CKW and CKR are obtained by means of analog phase-locked loops (PLL). A first PLL generates, from a quartz crystal oscillator and by means of a first voltage-controlled oscillator (VCO), frequency CKW. Frequency CKW is controlled by a horizontal synchronization signal Hsync of the received video signal. A second PLL generates, from frequency CKW and by means of a second VCO, frequency CKR. An analog disturbance signal is added to the control signal of this second VCO to render variable the control frequency of the VCO. This disturbance represents a predefined non-linear variation law which corresponds to the distortion desired for read frequency CKR.
Obtaining this disturbance signal is particularly difficult. Indeed, this analog signal has to represent, precisely, the desired variation law.
Moreover, the disturbance signal has to be synchronous with frequency CKR, and thus controlled by the horizontal scanning variable frequency, on the display side, since the variation law is repeated with a period corresponding to the duration of a line. This makes the implementation of the circuits for generating frequencies CKW and CKR particularly complex.
Another disadvantage of conventional conversion devices is that they are dedicated to one type of conversion. For example, if a device is meant to perform a non-linear expansion of the image for a conversion of images in format 4/3 to a display in format 16/9, the device used is dedicated to such a conversion.
Japanese patent applications JP-A-07067045 and JP-A-07298086 disclosure conversion devices using an analog signal to carry out a parabolic modulation of the read frequency. These devices have the above-indicated disadvantages.
Japanese patent application JP-A-07067046 discloses a conversion device comprising a circuit for controlling a memory that provides a read clock corresponding either to a clock provided by a PLL receiving the write clock, or to a pulse compressing the clock provided by the PLL. The switching between the two clock modes causes the image to be cut into three fixed areas.